Polyoma virus transformation of mammalian cells is associated with the appearance of new cell associated protein antigens which are of major importance in the immune rejection of such cells. In addition, these molecules likely provide the molecular basis for the induction and expression of some of the tumor cell-like properties associated with transformed cells. It is proposed to study the molecular nature of polyoma virus induced antigens in more detail with special attention being given to the antigenic changes occurring at the cell surface. This is to be accomplished by immuno precipitation, gel electrophoresis and autoradiography of radiolabeled antigens prepared from extracts of radiolabeled cells. Using similar techniques, we plan to explore the molecular basis for the observed in vitro cross reactivity between SV40 transformed and polyoma transformed cells which is presumably due to some immunologically similar antigen present on the surfaces of both types of cells. Studies of transformed cells of different species will likewise be carried out to obtain additional information on the surface antigens of transformed cells to determine which of these are specified by the species of transforming virus and which are influenced by species of cell origin. The in vivo activities of antigen preparations will also be explored.